1. Field of the Invention
The present invention relates generally to input devices for electronics and, more particularly, to a protection mechanism for force-based touch sensitive input panel or displays.
2. Description of the Background
Force-based touch screen systems (such as shown and described in U.S. Pat. No. 4,511,760 to Garwin et al. issued April 1985) are widely used in eBook readers, cellular phones and personal digital assistants (PDAs), PC Tablets, as well as laptops, PCs, office equipment, medical equipment, TVs Monitors, and other devices that use touch sensitive displays or panels.
In contrast to resistive, capacitive, infrared, Surface Acoustic Wave (SAW) or other more exotic touch screens, force-touch screen rely on pressure on or movement of a touch screen against underlying force sensors. They typically allow the touch surface to move a small distance in the z-plane (perpendicular to the touch surface) in order to ensure that the force is transferred completely without force additions, subtractions or delays to the underlying force sensors. There are commonly four force sensors placed in each corner of a rectangular touch panel. This direct transfer of touch force to the force sensors creates an inherent problem. Excessive force, or overload, can cause damage to the underlying sensors.
For example, in a typical application the HFD-500 Force Sensor From Hokuriku Electric Industry Co., Ltd., employs a small steel sphere (“ball”) as its active mechanical surface, seated in a piezo resistive sensor. The purpose of this ball is to allow for a perfect and friction free transfer of the applied force into the piezo resistive sensor. The HFD-500 Force Sensor is capable of micro-force detection with high sensitivity within a range of from 0-0.5 kilograms. Typically, the HFD-500 Force Sensor will reach its maximum electronic capability at approximately 2.5 kilograms of load, and will reach its failure point at approximately 4.5 kilograms of load. It should be noted that these specifications are per individual sensor, and the total loading on the touch panel may be significantly higher depending on the number of sensors and sensor preload, if any. Of course, different force sensors can tolerate different levels of maximum force loading. Nevertheless, most all can be temporarily or permanently damaged if exposed to extreme forces, such as external abuse of a Kiosk touch screen as a result of vandalism or even during transportation.
To date, there have been very few efforts to protect the sensors from overload forces.
One potential solution implemented in the MyOrigo™ SmartPhone circa 2003 used a flexible material (plastic) to transfer to force from the touch panel to the sensors. During an overload force, the plastic bends and transfers the force to the sensor housing or other fixed mechanical stop. Unfortunately, the plastic material also tended to bend during normal operation resulting in some applied touch force being stored as energy in the plastic material. This reduced the accuracy of the touch screen system. In addition, this approach was only practical for smaller touch screen units (up to about 5 inches of touch screen diameter) where the touch screen and other components could likewise be plastic. Larger screens tend to use a mix of different materials rendering this approach impractical, and so this approach imposes size constraints.
Another approach is to use a stop screw. Stop screws are typically placed near the sensors and are adjusted in height during the manufacturing process. Given foreknowledge of the sensor travel (the z-axis range of the ball within sensor e.g., most force sensors, such as the HFD-500 Force Sensor From Hokuriku Electric Industry Co., Ltd, are compressed by around 0.05 mm or more), the stop screw is adjusted accordingly to stop further compression before an overload force is reached. Conversely, stop screws allow completely free touch panel movement without interference during normal operation. Unfortunately, the height of the stop screws need to be extremely precise, and the adjustment is a costly and time consuming process. A typical stop screw must be adjusted to take up sensor load within a travel band of approximately 0.05 millimeters (0.002″). Positioning of the traditional stop screw is critical to insure the screw is able to carry overload in all typical use and abuse scenarios. These limitations make for difficult and time consuming setting procedures and limit the layout of the sensor in relation to needed stop screw positioning.
Moreover, any small change to the mechanical structure through aging, bending, wear, etc., can render the stop screws useless or interfere with the accuracy of the touch screen under normal operational forces.
The mechanical design of a force based touch screen system must allow for a close-to-frictionless movement in the z-plane to ensure that the complete force (F) of the touch is directly transferred to the force sensors. Any unknown disturbing forces, such as friction or bending would have a negative impact on the system accuracy. Interfering forces can be allowed for, as long as they are known, repeatable and can be compensated for.
With the foregoing in mind compressible materials have been added between the touch panels and sensor, such as Poron™ microcellular polyurethane pads, or other material which can be compressed without permanent deformation. These resilient pads allow for a longer travel distance of the touch panel before sensor overload force is reached. The longer travel distance in turn allows for the placement of mechanical stops (such as stop screws) with a less exacting tolerance. To an extent the interfering forces of the damping pad(s) are known and can be compensated for. However, the pads inevitably introduce force losses (during compression) and force increases (during expansion) which have a negative impact on accuracy. Moreover, the pad's multi-directional deformation can cause tilting of the touch panel and introduce additional force reading errors if the touch force is no longer perfectly perpendicular.
And so despite conventional mechanical solutions for integration different types of force overload protection in force based touch screen systems, the foregoing and all other known solutions tend to compromise performance, accuracy, economy of manufacturing, or all of the foregoing.
What is needed is a mechanical structure that allows for unrestricted transfer of z-axis forces from touch panel to sensor (without introducing any additional forces or movements into the system) during normal operation, dead-stop overload force protection to the sensors, and yet minimal additional cost of materials and/or manufacturing. The solution should also accommodate different sensors with different levels of maximum (destructive) force, different touch screen sizes, and should not impact overall product size. This invention described herein offers a simple and low cost solution to the above described problem.